The invention relates to a reversible storage system for electric energy, comprising a charging or discharging surface as the positive collector terminal and a charging or discharging surface as the negative collector terminal and a flow electrode with a pumpable dispersion comprising particles storing electric energy and at least one feed line and at least one drain line for the pumpable dispersion, as well as preferably a reservoir for the pumpable dispersion, corresponding dispersions, and methods, as well as other embodiments of the invention connected thereto.
Battery systems for storing electric energy via liquid media that can be pumped are known per se. Such a device is described, for example, in WO 01/03221, with this device comprising at least the following components: (a) a redox flow cell comprising (i) an anode in a catholyte chamber, (ii) a cathode in an anolyte chamber, and (iii) an ion-permeable diaphragm between these chambers; (b) two electrolyte containers, one container each for the cathode liquid (here also called positive electrode material, “catholyte-liquid” or “catholyte”) and a container for the anode liquid (here also called negative electrode material, “anolyte-liquid” or “anolyte”), as well as (c) a pumping system, which ensures the circulation of the cathode liquid and the anode liquid from the containers to the flow cell and back to the containers.
The principle of the flow cell according to WO 01/03221, which is also known as redox-flow cell, is based on the reduction and oxidation reaction of liquid electrolytes. According to WO 01/03221 they essentially comprise metallic salts dissolved in aqueous media. The cathode-liquid and the anode-liquid are pumped via a pump through the chambers of the flow cell, in which the respective electrodes are present. The electrodes typically comprise graphite/composite materials, with here graphite felts being used in order to increase the active surface and thus the power density. The two chambers, i.e. the catholyte chamber and the anolyte chamber, which are also called semi-cells, are separated by an ion-conductive diaphragm. Similar to fuel cells, the individual cells can also be switched serially to form a stack. Here, the electrodes acts as bipolar electrodes and the electrolyte flow occurs in parallel. Individual stacks can be electrically interconnected like conventional batteries, either parallel or serially in order to obtain the desired voltage.
A particular feature of this technology comprises that by the separation of the energy converter (i.e. the redox flow cell) and the storage medium (i.e. the two electrolyte containers) energy and power can be scaled independently from each other. Here, the amount of electrolytes determines the energy to be stored and the size of the active electrode surfaces determines the power of the battery. Due to the fact that the electrolytes are stored separated from the converter, practically no self-discharge occurs when the arrangement is idling. It only occurs when electrolytes are pumped through the cells.
The charged liquids (catholyte and anolyte) of the flow cells of prior art comprise dissolved metallic salts, which show a limited solubility so that this limits the energy density. The energy density of flow cells of prior art and/or redox-flow cells amounts to approximately 30-50 Wh/kilogram (kg), which is hardly more than achievable by conventional lead-acid batteries. Additionally, the aqueous electrolyte solutions lead to precipitations at and corrosion of conductive parts. Thus, the redox-flow cell allows the storage and refilling of electric energy in a liquid form, however for example vehicles must carry a much larger amount of liquid in order to travel an appropriate distance.
US 2010/0047671 describes how, instead of solutions of electrolytes, in an appropriate manner dispersions of particles, which may also include electrolytes, can act as anolytes and/or catholytes in respective flow chambers. However, in the examples shown a strong internal resistance must be expected, because the conductive electrolytes (collectors) are arranged far apart from each other.
Batteries in the form of static storage systems for electric energy are also known, such as lithium batteries (Li-batteries). Such a battery is described for example in EP 0 880 187 B1 and comprises the following components: (a) an anode material on a collector film, (b) a cathode material on a collector film, (c) an ion-permeable separator film, as well as (d) an anhydrous electrolyte solution.
In the Li-battery the anode material and the cathode material are each applied on an electrically conductive collector film. Both films are separated from each other by an ion-conductive separator film and immersed in an electrolyte solution. In order to allow storage surfaces as large as possible in a small volume, these films are wound or folded.
In the Li-battery an energy density of 140 Wh/kg is achieved by the electrochemical elements included, which is only sufficient for a driving range of approx. 130 miles (200 km) in an average passenger vehicle. Additionally, Li-batteries are very expensive by the complicated production of the storage films they comprise and can easily be destroyed by exterior influences. For example, if vibrations lead to a damage of the integrated film, this may lead to short circuiting and thus to the destruction of the cells. A destruction of the cells can also occur by an excessively fast charging or discharging process when the thin films burn through, for example. Many materials for electrodes, which might lead to a higher storage capacity, cannot be used because they cannot be sufficiently fixed on the carrier film. For example, when silicon is used as the anode material it would lead to a ten-fold higher storage capacity than the relatively frequently used graphite; however, the volume of silicon increases considerably during the charging process so that it cannot be fixed permanently on the carrier film. The storage capacity of conventional batteries is also limited by the structural size of the cells. Additionally, a single charging process takes several hours.
Further, capacitors are known for the storage of electric energy, particularly electrolyte capacitors and double-layer capacitors. Capacitors comprise two electrolytes with their distance being as short as possible and with an area of non-conductive, insulating features being therebetween (dielectric). In an electrolyte capacitors, also called “elco”, a non-conductive insulating layer is created on the metal of the anode electrode by way of electrolysis, which forms the dielectric of the capacitors. The electrolyte forms the cathode (counter-electrode). It may comprise a liquid or a pasty electrolyte (ion conductor) or a solid electrolyte (electron conductor). Most recent developments are the polymer-electrolytes. It is distinguished between three types of design, which include as the dielectric (a) aluminum oxide, (b) tantalum-oxide, or (c) niobium-pentoxide.
Double-layer capacitors are also known (e.g., trade names Gold Cap, Supercap, Ultra Cap, Boost Cap). They are characterized in a high energy density. Their relatively high capacity is based on the dissociation of ions in a liquid electrolyte, which form a thin dielectric of a few atomic layers at the boundary layer to the electrodes. The principle of energy storage in the electro-chemical double-layer (Helmholtz-layer) is known per se. The relatively high capacity is based on the low thickness of the double layer. Frequently, carbon is used as the material of the electrodes in various modifications: Aqueous electrolyte solutions can be used as electrolytes, however, primarily organic electrolytes are used based on quaternary salts, such as tetra ethyl ammonium borofluoride (TEABF), dissolved in acetonitrile or propylene carbonate. Organic electrolytes allow a cell voltage of typically 2.5 V. Aqueous electrolytes, such as potassium hydroxide (KOH) or sulfuric acid (H2SO4) have a nominal voltage of maximally 1.2 V, however, due to the considerably lower interior resistance they have a higher capacity. The design of double-layer capacitors differs only slightly from that of batteries. They primarily relate to mono-polar arrangements, in which the electrodes are wound or stacked.
Capacitors, particularly double-layer capacitors also used as energy storage for electric energy due to their higher energy capacity, are limited with regards to their capacity due to the statically arranged charge carriers (wound films). Presently, double-layer capacitors only achieve an energy density of 4-20 Wh/kg.